In order to clarify the role of the immune response following meniscal allo-trnsplantation, we orthotopically transplanted meniscal allografts in rats. The experiments were designed to examine the possibilities of meniscal allograft transplantation ; also the effects of cryopreservation on the grafts in techniques which used immunologically minor mismatched rat groups(Fistier 344 and Lewis). Three minor mismatched groups of rats received ;1.16 fresh, meniscal allografts without immunosuppression.2.6 fresh meniscal allografts with immunosuppression.3.8 cryopreserved meniscal allografts without immunosuppression.In 12 of the Fisher 344 rats, the menisci were transplanted as isograft controls.The transplanted allografts were evaluated histologically at various intervals from 4 to 21 weeks and at one year in the isograft controls. The isograft meniscal transplantations were all successful at one year. In the immunosuppressed rats, the fresh allograft appearances resembled those of the isografts. However, the fresh allografts behaved very differently from the isograft group in non-immunosuppressed rats. The allografts were invaded by synovial tissue and became virtually scar tissue by 21 weeks. However, cryopreserved meniscal allografts seemed to be completely preserved and did not show any degenerative changes at up to 21 weeks after transplantation in rats without immunosuppression.